40 Ar/ 39 Ar dating and paleomagnetism of the Miocene volcanic succession of Monte Furru (Western Sardinia): Implications for the rotation history of the Corsica-Sardinia Microplate

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40 Ar/ 39 Ar dating and paleomagnetism of the

Miocene volcanic succession of Monte Furru (Western

Sardinia): Implications for the rotation history of the

Corsica-Sardinia Microplate

A. Deino, J. Gattacceca, R. Rizzo, A. Montanari

To cite this version:

A. Deino, J. Gattacceca, R. Rizzo, A. Montanari. 40 Ar/ 39 Ar dating and paleomagnetism of the

Miocene volcanic succession of Monte Furru (Western Sardinia): Implications for the rotation history

of the Corsica-Sardinia Microplate. Geophysical Research Letters, American Geophysical Union, 2001,

28 (17), pp.3373-3376. �10.1029/2001GL012941�. �hal-03036435�

GEOPHYSICAL RESEARCH LETTERS, VOL. 28, NO. 17, PAGES 3373-3376, SEPTEMBER 1, 2001

4øAr/39Ar

dating and paleomagnetism

of the Miocene

volcanic

succession

of Monte Furru (western Sardinia): Implications for

the rotation history of the Corsica-Sardinia microplate

A. Deino

1, J. Gattacceca

2, R. Rizzo

3, A. Montanari

4

Abstract. Although it is widely acknowledged that the Corsica-Sardinia microplate rotated counterclockwise with

respect to Europe during Oligocene-Miocene time, the precise

timing of this event has yet to be determined. We have

measured the age and degree of rotation of a single 'tie-point'

in the rotation history of the microplate. Biotite and sanidine

4øAr/39Ar

age determinations

of the 200m-thick Monte Furm

obtained for the 12 volcanic flow units comprising this succession indicates that by 18.2 Ma Sardinia remained 13 ø shy of its final rotation angle. These results demonstrate that

the movement of the Corsica-Sardinia block and the opening of the liguro-provenqal basin terminated later than previously

estimated based on paleomagnetic and geochronologic studies

of Sardinian volcanic rocks, in agreement with paleomagnetic

data from Sardinian sediments.

Introduction

Accompanying the opening of the liguro-provenqal basin, the Corsica-Sardinia microplate drifted eastwards to its current position during Oligo-Miocene, based on various palinspastic reconstructions for this part of the Mediterranean (e.g. [Mauffret et al., 1995]). However, the timing of this event (initiation, rate of rotation, and cessation) remains uncertain. Paleomagnetic investigations of Sardinian volcanic rocks document a NW to N shift in declination, interpreted as

evidence of rotation of a rigid Corsica-Sardinia microplate. By

analyzing paleomagnetic and K/Ar geochronologic data from these rocks, Montigny et al. [1981] concluded that the 30 ø rotation of Sardinia-Corsica occurred in the Lower Burdigalian,

between 21 and 19.5 Ma (ages re-calculated with decay

constants from Steiger and Jiiger [1977]). However, statistical evaluation of the pre-existing paleomagnetic and geochronological data on volcanic rocks by Todesco and Vigliotti [1993] and Speranza [1999] showed while the onset of rotation could be constrained to -21.5 Ma, termination

could not be accurately assessed primarily due to uncertainties

introduced by secular variation. Vigliotti and Kent [1990] and Vigliotti and Langenheim [1995] investigated Miocene

sedimentary rocks from Corsica and Sardinia respectively.

Results from Corsica were not deemed reliable, but those from

1 Berkeley Geochronology Center, CA

2 CGES-Sedimentologie, Ecole des Mines de Paris, France

3 DIGITA, Cagliari, Italy

40sservatorio Geologico di Coldigioco, Italy

Copyright 2001 by the American Geophysical Union.

Paper number 2001GL012941.

0094-8276/01/2001GL012941505.00

Sardinia indicated that rotation was incomplete prior to

15 Ma. Speranza et al. [1999] investigated Miocene

sedimentary rocks from Sardinian, and documented a 20 ø rotation of Sardinia subsequent to 18.4 Ma. Thus there exists a

discrepancy in the timing of cessation of rotation as inferred

from studies of volcanic and sedimentary rocks.

Paleomagnetic analyses of 16 sites spanning the Miocene

eruptive succession of Monte Furm demonstrated an apparently gradual clockwise rotation of declination from 2900-340 ø at the base to 350ø-015 ø at the top [Assorgia et al., 1994]. These paleomagnetic data were interpreted as possible

evidence for the counterclockwise rotation of Sardinia.

Subsequently,

we have acquired

4øAr/39Ar

ages and new

Monte Furre, in a well understood stratigraphic setting, to

help constrain the timing of the tectonic rotation of Sardinia.

Geological setting

-1000 m of volcanic rocks of upper Oligocene to lower Miocene are exposed near Bosa on the west-central coast of

Sardinia. This succession is composed of basaltic to andesitic

lava flows, domes, and interbedded pyroclastic breccias, and of intermediate to rhyolitic, predominately pyroclastic volcanic

products including flow, fall and surge deposits. The

uppermost 200 m of the volcanic succession is well exposed at Monte Furm (40 ø 17'N, 8ø30'E), where the following

stratigraphy has been established, from base to top: a) the

Great Ignimbrite of Bosa (GIB), a rhyodacitic (Qtz+Pl+Bt_+Kfs_+Hbl) ignimbrite, composed of three flow units (designated BO1, BO2, BO3) attaining a total maximum thickness of 100 m, b) a rhyodacitic to dacitic

(Pl+Cpx+Bt_+Hbl_+Qtz) composite ignimbrite ~70 m thick

(four flows, MF1-4), c) a 20 m thick ignimbrite comprised of two flow units (IF1, IF2), and finally d)a 15 m thick dacitic (Pl+Bt+Cpx) welded ignimbrite which forms the summit

plateau of Monte Furre. An andesitic (Pl+Opx+Cpx_+Hbl) dike/dome and a dacitic sill were emplaced contemporaneously

with accumulation of the ignimbritic succession. No tectonic

tilt is observable at Monte Furru.

Paleomagnetism

Paleomagnetism of the Monte Furru succession was

previously investigated by Assorgia et al. [1994]. They

concluded that the overall characteristic remanent

magnetization (ChRM)declination pattern demonstrates a

progression from NW to N directions from the base to the top

of the succession. Some unanswered questions were raised by

this study, such as the nature of an unusually large declination

range (309 ø to 7 ø) in the lower flow (BO1) of the GIB, and the

implications of some sites with anomalously low

s E

BO3 flow ½

core coordinates _• | ? BO2 flow

ARN

=

230

mA/mJ

15

mT ! • core

coordinates

m 1½45 mT I N

45 I '"-• mT

Down • • • t N E IDown • NRM

Figure 1. Orthogonal demagnetization plots of some samples from Monte Furru succession.

inclinations. In addition, some directions from Assorgia et al. [1994] were obtained from only a small number of samples. The questions and concerns regarding the paleomagnetism of volcanic units at Monte Furm led to the acquisition of additional paleomagnetic data as part of this investigation.

The sequence of 12 volcanic flow units at Monte Furru was sampled with solar oriented drill cores along multiple vertical profiles, to avoid local magnetic anomalies. Natural remanent

magnetization was measured with a JR4 Agico spinner

magnetometer. Alternating field and thermal stepwise demagnetizations were employed with identical results. Demagnetization orthogonal plots and principal component analysis assisted determination of ChRM. Fisher [ 1953] statistics were used to process the data. For all the samples, magnetic saturation occurred at 150-300 mT and the Curie temperature is-550-580øC, indicating titaniferous magnetite as the main magnetic carrier. Hysteresis loops indicate pseudosingle domain grains. For almost all samples, ChRM was isolated after removing a viscous and occasionally a secondary magnetization (Fig 1). The uppermost dacitic flow, struck by lightning at the sampling locations, required the use of the remagnetization circles method. Different sites within a given flow consistently gave indistinguishable directions at

the 95% confidence interval. A mean direction for each flow

using the sample data from each site is shown in Fig 2 and Table 1. Though directions of the upper flows are somewhat

closer to N than those of the lower flows, the results do not

indicate a definitive declination trend as described by Assorgia et al. [1994]. We suspect that the discrepancies between our studies may at least partly be explained by the sampling technique of Assorgia et al. (i.e., all cores for a given site derived from a single orientated block).

Geochronology

Sanidine occurs only in the GIB whereas biotite was found

throughout

the succession.

Two variants of the nøAr/39Ar

technique were employed, both using an Ar-ion laser as the sample heating device: sanidine phenocrysts were dated individually by the focused-beam, total-fusion technique, while biotite phenocrysts in small populations (5-15 grains) were incrementally heated using a defocused laser beam ([Deino and Potts 1990; Deino et al., 1990; Deino et al., 1998] for analytical details). Sanidine from the Fish Canyon

Tuff of Colorado was used as the neutron fluence monitor, with

a reference age of 28.02 Ma [Renne et al., 1998].

Five sanidine samples from different stratigraphic levels in the GIB were dated. Age-probability density diagrams of the dating results (Fig 3) indicates unimodal, gaussian-like distributions for each sample (MSWD's range from 0.72-1.1). The five concordant mean sample ages yield an overall weighted mean age of 18.18 + 0.03 Ma (this excludes external errors in decay constants and the age of the standard, which

would

increase

the overall error

estimate

of the 4øAr/39Ar

ages

10ow 0' 10OE

2'

".

••,,

',.,

•.,,

'.'

s,6.'•i• 50-

Figure 2. Paleomagnetic directions of flows of the Monte Furru sequence with tx9s confidence limits. The direction computed from the Eurasian pole is indicated. Numbers refer to the stratigraphic order of flows.

by -2% lo, [Min et al., 2000]) This well-constrained result serves as the reference age to which the biotite dating results on the GIB and overlying units can be compared.

Biotite incremental-heating spectra are shown in Fig 4 with integrated and plateau ages indicated; plateau ages are also shown in Fig 5 in relation to the stratigraphy. Two aliquots of a biotite separate from each of five samples were dated. Most of the experiments yielded similar spectra, demonstrating

concordance

throughout

the bulk of the 39Ar

release.

However,

a common tendency consisting of a stair-step decrease in the age of steps within the initial 5-30% of gas release was noted for most spectra. In a few cases, this was also accompanied by a clearly defined stair-step increase in age in the final 10-30%

of 39Ar

release.

Two explanations

are the most likely: excess

4øAr

(a common

interpretation

of 'saddle

shaped'

spectra),

or

alteration.

Excess

4øAr

appears

unlikely in light of the high

temperature of emplacement of at least one of the flow dated. Sample MF-11, which yielded spectra that are typical of this biotite suite, is from the uppermost, densely welded flow. It was therefore emplaced above -600øC, the minimum welding temperature evaluated for a 15 m thick pyroclastic flow [Riehle, 1973], well above the blocking temperature of biotite (variable but likely less than 350øC, [Harrison et al., 1985]).

This suggests

that excess

4øAr

entrapped

in the biotite prior to

Table 1. Paleomagnetic directions at Monte Furru

Flow D ø I ø k c•9s ø n/n o NRM K 8 ø DA 351.4 38.6 200 3 13/13 14.0 20.3 15 Dike 9.2 49.8 127 4.3 9/9 1.44 35.0 17 Sill 338.1 55.6 826 4.3 6/9 0.56 13.7 10 IF2 0.2 50.6 110 4.6 9/10 2.49 6.4 9 IF1 354.6 51.2 118 4.2 10/10 1.98 5.8 6 MF4 348.6 62.0 908 1.4 11/12 1.64 3.3 7 MF3 353.2 61.4 114 3.9 12/12 0.44 5.6 6 MF2 350.6 64.1 120 4 11/12 0.39 5.1 9 MF1 351.0 42.1 235 4.5 5/5 0.17 6.2 15 BO3 340.2 63.1 131 3 18/22 0.58 3.0 11 BO2 341.2 61.7 229 2.3 18/21 0.14 3.0 10 BO1 344.9 58.3 99 2.2 43/47 0.22 3.3 5 Mean 1 351.0 55.2 62 5.3 12 Mean 2 351.1 52.9 51 7.3 8

k: Fisher precision parameter, n/no: number of samples used in

analysis/number of measured samples, NRM in A/m, K:

susceptibility in 10 -3 SI, 8: angular distance between the VGP and

the mean of the VGP. All VGP are non-transitional. Mean 1/2: mean

direction computed with individual flows/by combining flows BO2-3,

60 Moles 39At = 40 [x 10 -14] ß ß .

o ...

.

...

i

4o

1

ß

. ,•..', J '•,g,

?," .. ß ,

-'-•'-' '"x MF'-I [ MF~3-...-'- ... / .... --,,. ,a-"' • .... -, ... ,o.o MO-2 ,,/,•/.//";•.•."'" ' ... '• ... <"•'-"• 18.22 + 0.06 [- • ..,•./....-j;.--'< ; - : 18.15 + 0.06 ""-•'--.:-.•-., r •---•'-••'i"•. ' .:.. :..• .... : ;•.,•, ,o.o•. •-•.,..-5:.' ::•..•-•....-.• ,-•--J. 18.00 18.05 18.10 , 18.15 18.20 18.25 18.30 18.35 18.40 Age (Ma) 100 98 96 94 4O 2O 0

Figure 3. Age-probability density spectra for the single-

crystal •øAr/•øAr

total-fusion

analyses

of six GIB sanidine

samples. The weighted-mean age with l c• standard error of the mean is shown for each sample; the outermost error bars show uncertainty including error in the neutron fluence parameter, J, (0.3 %). The two biotite plateau ages of the GIB are shown.

eruption would be rapidly lost during post-emplacement cooling of the flow. It is more likely that the discordance of

the early steps,

and of some of the final steps,

records

alteration of the biotite at some stage subsequent to crystal

growth (conceivably as early as during pre-eruptive conditions in the magma chamber, or as late as surficial weathering of the outcrop). The alteration process has resulted in the preferential

removal

of K relative

to 4øAr*,

leading

to artificially old ages.

The extent of this alteration process and its influence on the biotite plateau ages is fairly minor, however, as deduced from

several lines of evidence: 1) Spectra are concordant throughout most of the gas release in all incremental-heating

experiments,

2) Radiogenic

contents

are usually

>95% •øAr*,

1oo

._.21 ... %4OAr' , ... %4OAr- , ... r]_-I %4OAr.

Sample MF-11 Sample MF-11 Sample MF-6

= 18.28ñ0.07Ma

-- L, 18.31+0.07Ma

I i

• 18.25

+

0.07

Ma

Integrated Age = 18.37 +0.07 Ma Integrated Age = 18.38 + 0.06 Ma Integrated Age = 18.28 + 0.06 Ma

•20 •,19

16

%4OAr* %4OAr* %4OAr*

Sample MF-5 Sample MF-4 Sample MF-12

18.27

+

0.07

Ma

• [ 18.26

+

0.06

Ma

18.28

+

0.07

Ma

J _ r • l' . •.

Integrated Age = 18.31 +0.07 Ma Integrated Age = 18.31 +0.06 Ma Integrated Age = 18.31 +0.06 Ma

:1oo •.2o •19 .,.. m:18 .17 1% 20 40 60 80 100 0 20 40 60 80 1000 20 40 60 80 100

Cumulative %39Ar Released Cumulative %39Ar Released Cumulative %39Ar Released

Figure 4.

Incremental-heating

apparent •øAr?øAr

age

spectra from biotite phenocrysts. Apparent-age uncertainties of the individual steps are shown at 2c•, uncertainties in the plateau age and integrated age are shown at l c•. Incremental

heating steps yielding less than 1% of the total •øAr were

excluded from the data set. Plateaus were determined on the

following basis: 1) Steps falling entirely within the first and last 10% of gas release were excluded, 2) steps at the ends of plateau segments deviating in age more than 1.8 times the weighted mean plateau age were excluded, 3) the plateau must have a minimum of three contiguous steps, 4) the plateau must

have at least

50% of the total •øAr

release,

5) the MSWD of the

plateau ages must not exceed that explained by analytical

scatter alone at the 95% confidence level.

MF-11 2OO B 18.31 ñ 0.07 B- biotite S' sanidine 150 MF-6 B 18 25 ñ 0.07 B 18.20 ñ 0.08•,,,- MF-5 B 18.25 ñ 0.08 B 18.27 ñ 0.07 •"' MF-4 100 B 18.26 +_ 0.06 •" B 18.26 _+ 0.06 MF-3 S 18.15 + 0.06•..- MF-2 S 18.14 +0.06? MF-1 S 18.22 ñ 0.06•,-- MF-12 S 18.20 + 0.06 B 18.28 + 0.06 B 18.29 ñ 0.07,,- m MO-2 S 18.19__0.06 *"'0 ,. ,,, ,,;

40Ar/39Ar Ages v -. ',•. ,• •'•! Andes•les J 330 340 350 0 10'E

(Ma :t: 1•) Unit Declination (•-ot9s/cosl)

Figure 5. Age and paleomagnetic declinations of the

volcanic succession of Monte Furru. Dashed line indicates the

declination computed from the European paleomagnetic pole.

3) Microprobe analyses of biotite from the GIB yields a K20

content of 8.5%, indicative of fresh igneous biotite composition, 4) Small-population aliquots pairs from each of five samples are concordant in plateau and integrated ages, indicating homogeneity in the sample separates, 5) Two aliquots of biotite from the GIB (sample MF-12) yielded plateau ages of 18.28 _+ 0.07 and 18.29 _+ 0.07 Ma, statistically indistinguishable from the sanidine reference age of 18.18 _+ 0.03 Ma obtained from the same sample at the 95% confidence level, 6) The complete range of biotite plateau ages is just 110 ka (18.20 + 0.08 to 18.31 _+ 0.07 Ma), and all are statistically indistinguishable from each other. Weighted mean ages derived from the biotite plateaus for the three units dated (18.29 _+ 0.05 for the GIB, 18.25 _+ 0.03 for the Monte Furru ignimbrite, and 18.30 _+ 0.05 Ma for the capping dacitic flow) differ by less than 50 _+ 120 ky (2c•).

Discussion

Conservatively, the biotite chronostratigraphy can be summarized by stating that the 200 m-thick Monte Furru volcanic pile likely accumulated within less than 200 ky. This

relatively short accumulation interval precludes explanation of the variation of paleomagnetic directions by tectonic rotation during emplacement. Instead, the paleomagnetic

scatter must be attributed entirely to paleosecular variation of the geomagnetic field. Our dataset displays a 26 ø range in

inclination and a 31ø range in declination, compatible with

normal secular variation pattern.

A mean direction encompassing all Monte Furru flows can be calculated with the aim of averaging secular variation. The apparent polar wander path for Eurasia gives a lower Miocene pole located at 84øN, 156.3øE, A95=2.6 ø (Besse and Courtillot

[in press]), predicting for Monte Furm a paleomagnetic

direction D=3.9 ø, 1=54.6 ø. The measured mean inclination

(55.2 ø ) agrees with the predicted value, although our

declination implies a 13 _+ 7 ø counterclockwise rotation with respect to Europe (95% confidence limit calculated from

3376 DEINO ET AL.: 40AR/39AR DATING AND PALEOMAGNETISM OF SARDINIAN VOLCANICS

Demarest [1983]). This calculation may be potentially refined

by combining related flow directions before computing an

average direction for a given timeframe. Several sets of successive flows with near identical lithologies bear indistinguishable paleomagnetic directions (BO2/BO3, MF2/MF3/MF4, IF1/IF2), suggesting that the flows in each set were emplaced within a short interval relative to the rate of secular variation. To avoid over-representing these sequences in an overall average, we combined these directions and calculated a new mean for the succession (mean 2, Table 1).

This mean direction remains close to that calculated without

averaging related flows, and again implies a 13 ø rotation with no latitudinal movement. Hence our results suggest that by 18.2 Ma Sardinia had not yet completed its rotation with respect to the Europe and a rotation of-13 ø was forthcoming. Scatter of the Virtual Geomagnetic Poles (VGPs), defined as the angular standard deviation (ASD) of the VGPs, can be used to help evaluate whether secular variation has been sufficiently averaged. The between-site ASD is 11.6 ø (8.8- 17.1 ø 95% confidence limits). Due to the small number of VGP (eight) used in the calculation, this estimation remains tentative. We note that this value is slightly lower but still compatible with the 15-16 ø that is expected at the latitude of Sardinia [McElhinny and McFadden, 1997]. In addition, the mean paleomagnetic inclination is identical to that computed from the Eurasian pole, suggesting that paleosecular variation has been successfully averaged by this dataset.

Conclusion

The Monte Furm volcanic succession was emplaced within less than 200 ky, beginning at 18.18 + 0.03 Ma. This rapid emplacement interval dictates that the variation in the paleomagnetic declination record be attributed to the influence

of secular variation and not to tectonic rotation. The overall

mean paleomagnetic direction of the volcanic succession indicates that by -18.2 Ma Sardinia remained -13 ø shy of its final rotation position.

Our results disagree with conclusions drawn from previous paleomagnetic-geochronologic studies on Sardinian volcanic rocks which placed the end of rotation at 19.5 Ma [Montigny et al., 1981]. We believe that the results of this study may have been hindered by insufficient accommodation of the effects of secular variation as well as inaccuracy of the K-Ar dating. Our results, however, are in accord with those derived from paleomagnetic investigation of Sardinian sediments [Vigliotti and Kent, 1995; Speranza et al., 1999].

Finally, it can be noted that all units at Monte Furru have normal polarity and were likely emplaced during chron 5En. The younger age limit for this chron of 18.281 Ma given by Cande and Kent [1995] is in slight conflict with the mean sanidine age reported here (18.18 + 0.03 Ma). Either the chron

age is miscalibrated or the sanidine age is slightly too young.

Ackowledgments. JG thanks B. Henry and M. Le Goff

(IPGP) for assistance. AM was supported by the Coldigioco

Research Fund. AD and AM received funding from Exxon. We thank F. Speranza and an anonymous referee for their review.

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(Received January 01, 2001; revised June 14, 2001;